Plasma energy extraction



Aug. 19, 1969 G. L. CANN ET AL 3,462,622

I I 1 PLASMA ENERGY EXTRACTION Filed April 27. 1966 v s Sheets-Sheet 1 FIG. 1

7 INVENTORS. v GORDON L.CANN ROBERT L.HARDER BY P UL JACOBS 5t et m A TTORNEYS Aug. 19, 1969 .G. L. CANN E L' 3,462,622

PLASMA ENERGY EXTRACTION Filed April 27. 1966 s Sheets-Sheet 2 INVENTORS GORDON L. CANN ROBERT L. HARDER PA F COBS bl a A T TORNEVS Aug. 19, 1969 G. L. CANN ET AL I 3,462,622

I PLASMA ENERGY EXTRAC ION I 3 Sheets-Sheet If Filed April 27. 1966 FIG.

INVENTORS. GORDON L CANN U ROBERT L.HARDER PAU F. JACOBS United States Patent Q 3,462,622 PLASMA ENERGY EXTRACTION Gordon L. Cann, Laguna Beach, Robert L. Harder, Altadena, and Paul F. Jacobs, South Pasadena, Calif., asslgnors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Apr. 27, 1966, Ser. No. 545,702 Int. Cl. H02k 45/00 U.S. Cl. 310-11 7 Claims This application relates to plasma containment devices and more specifically to means and methods for extracting electrical energy therefrom. The present invention is particularly adapted for use with plasma containment devices of the type more fully described in the present joint applicant, Gordon L. Canns applications 457,414 and 457,746 filed respectively on May 20 and 21, 1965. Reference should be made to those applications for a more complete description of the containment device itself.

The energy in a contained high energy plasma must be dissipated in some manner Whether the energy represents only electrical input energy or is enhanced by chemical or thermonuclear reactions within the plasma. Ordinarily the energy is extracted in the form of thermal energy, but it is desirable to extract some of the energy as electrical energy to reduce the heat which must be dissipated by internal elements of the plasma containment device and also to provide a simpler method of utilizing the energy contained in the hot plasma.

It is accordingly the principal object of this invention to provide improved means and methods of directly extracting electrical energy from a plasma containment device. Further objects will appear in connection with the description which follows.

FIGURE 1 is a highly simplified cross sectional view of apparatus embodying the invention.

FIGURE 2 schematically illustrates a different form of the invention.

FIGURE 3 shows a schematic sectional view of a simplified version of the FIGURE 1 apparatus.

FIGURE 1 shows a portion of an illustrative embodiment of the invention, the omitted right hand portion of the figure being a mirror image of the portion shOWn. There is shown a chamber 10, which is evacuated by vacuum pump 12, and which illustratively contains a set of three magnet coils 14, 16, and 18, each of which is enclosed by a water cooled shield 19, 20, and 21, the Water cooling means not being shown. Each shield is electrically insulated from its respective magnet but is electrically conductive on its outer exposed surface. Each magnet coil is connected to a DC power supply 22, 24, 26, respectively. Electrode assembly 36, is located on the axis of the magnetic field and includes a central cathode 40, surrounded by an anode 42. Channels are provided for entrant and exit of coolants to and from the cathode and anodes; for purposes of simplicity, however, these channels are not herein shown. Further details of these and other features of the electrode assembly may be found in the Cann applications previously alluded to, and also in the present joint applicants application 458,837, filed May 20, 1965. Ionizable material is introduced into chamber 10, through feed passage 44, which is connected to a supply tank 46. A power supply, 48, provides an arc current between cathode and anode.

When pump 12, and the various power supplied are energized and material is introduced through feed channel 44, a rotating column of high temperature plasma is formed which is schematically indicated by reference character 80, a representative ion trajectory being suggested by reference character 11. The radial electric field ice existing between cathode 40 and anode 42 has been shown in the Cann applications alluded to, to propagate down the length of plasma column 80, and it has further previously been shown that the equipotential lines in the plasma column should, in a general way, conform to the magnetic field lines. It has now been found, however, that the electrical potential at the outside of the plasma column can substantially exceed the anode potential. Magnet shields 19, 20, and 21, are in contact with the outside of the plasma column and tend to be charged to this higher local plasma potential. It will be understood, of course, that the plasma column does not have a sharply defined outer surface as might be suggested by the figure.

External loads 50, 52, and 54, individually connect magnet shields 19, 20, and 21, to anode 42. The total load 55 can be simple heat dissipating resistors or can be utilization devices such as, motors or DC-AC power convertors. If the individual loads are of infinite resistance or are removed entirely, the magnet shields associated therewith will simply absorb heat from the plasma column. With a finite value of load, electrical power will be extracted from the plasma column by the heat shield and be dissipated in the external loads. The amount of energy extracted by each magnet shield can be varied by adjusting the value of the corresponding load, there being an optimum load for each operating configuration which will extract the maximum amount of electrical energy. The loads can be adjusted to apportion the extraction of electrical energy between one or more magnet shields in any desired way. Similarly, increasing the inside diameter of the magnet shields will increase the open circuit voltage, but reduce the current which can be drawn. Controlling the load impedance controls the shield voltage, and also controls the ion impingement velocity.

There are at least three mechanisms by which the plasma column can become more positive than the anode potential. First, a radial number density gradient of ions exists in the plasma column. This gives rise to a potential drop of the form where n =ion density at the anode radius, n equals ion density at the inner collecting surface of the magnet shield, KT/[e|=energy in electron volts of the ion, and z charge on the ion.

Second, the ions are rotating. Interaction of the rotating ions with the applied longitudinal magnetic field establishes a potential drop of the form where w=an average angular velocity of the gas, B =strength of the applied magnetic field, R =radius of the collector, and R =radius of the anode.

Third, the ion rotation produces a potential drop through a centrifugal separation of ions and electrons of the form m,,w R 3 2z|e| where m =ion mass, and the other symbols are as previously defined.

When no current is drawn from the magnet shield the overall potential will build up to a value When current is drawn from the shields, it is supplied mainly by a differential outward How of plasma ions.

Where a chemical or thermonuclear reaction takes place in the plasma, the voltage on the magnet shield will be increased due to an increase of the term T in the equation for V When the ions collide with the inner surface of the magnet shields, they have only a small radial velocity and do not sputter the surface, but rather combine with electrons and form atoms which can then be pumped out of the system.

FIGURE 2 is a schematic sectional view of a modified embodiment of the invention. In this depiction both ends of the symmetrical device are shown but the midpoint is partially deleted as such section is essentially a continuation of the structural portions it adjoins. Chamber of FIGURE 1 has been replaced by a composite structure including a central tube 150, connected by an insulator 152, to an end chamber 154. Magnet coils 156 are positioned externally to 150. Electrode assembly 236 is positioned in end chamber 154 and anode 42 is covered by a water cooled shield 94 which is further described in an application by the present applicants filed approximately simultaneously with the instant one and entitled Plasma Arc Electrodes With Anode Heat Shield.

Tube 150 is formed with a series of ports or plenums which are seen to be partially defined by circular restrictors provided between some of the magnet coils. The mid-section of tube 150, not shown, may or may not include further restrictors. The circular restrictors, preferably water-cooled metal, are seen to be adjacent each port on the side of the port nearest the outer end of 'the device. The outer restrictors 200 have the smallest internal diameter, intermediate restrictors 202 have a larger diameter, and inner restrictors 204 have the largest diameter. Each plenum or port is connected with its own vacuum pumping system, except the corresponding symmetrically positioned ports may be connected to a common pump as shown. Thus plenums 205 are connected to a common pump 110 and plenums 206 are connected to a common pump 208. In a similar manner the two outlets serving plenum 210 are connected to a common pump 212 and the two outlets serving plenum 214 are connected to a common pump 216. Each restrictor intercepts a portion of the longitudinal flow of plasma along the outside of the plasma column contained in tube 150 during operation of the device. The plasma recombines and is neutralized at the surface of the restrictors and is removed through the associated pumping system. In this way the total pumping effort required to maintain the vacuum chamber evacuated is distributed over a large number of pumps. The distribution of pumping load among the various pumps can be controlled through variation of the diameters of the various restrictors. Thus, in FIGURE 2 pump 216 must be adapted to pump against a low suction port pressure. However, the pump inlet pressure will be progressively higher at pump 212 and 208 and will be highest at pump 110 which will also handle the largest mass flow. In this way the material necessarily introduced into the plasma through feed passage 44 is removed in the most efficient possible way. Furthermore, much of the thermal energy in the plasma which would otherwise be dissipated at or near the electrode assemblies is removed by the various restrictors. By spreading the thermal load over more elements, the load on each one is reduced and operation at higher power levels becomes possible. Additional features and advantages of maintaining pumping action in accord with the teaching illustrated in this figure may be gained from examination of the applicants, Cann and Harders, approximately simultaneously filed application entitled Multi-Level Vacuum Pumping System for Plasma Containment Device. It will, however, be noted that in connection with the teaching of the present invention, the several restrictors 200, 202, and 204 may be connected through external loads 50, 52, and 54 respectively, to anode 42 to thereby serve a function similar to that performed bymagnet shields 19, 20, and 21 in FIG- URE 1. By means of such an arrangement the restrictors not only assist in more efiectively evacuating the plasma device, but serve as well to extract electrical energy.

FIGURE 3 shows a schematic sectional view of a simplified version of the apparatus shown in FIGURE 1. Only coil 16 and shield 20 are employed in this version. The magnet coil 21 that surrounds anode 42 is not of course within the plasma stream but acts rather to extend the magnetic field through the electrode assembly in a manner suggested by the B field lines depicted in the diagram. An adjustable power source 23 is provided for coil 21 in order to provide control of the coil current and thereby achieve a degree of control over the longitudinal field convergence. As in FIGURE 1 a representative ion trajectory 11 is shown and in addition current density flow paths are schematically suggested both for the cathode-anode path and for the radial drifting ions impinging on shield 20. The dimensions used in a particular experiment are tabulated below:

Dimension: Value cm.

A 7.5 B 1.5 C 4.0

The magnets were energized so as to produce an axial field strength of approximately 3,000 gauss with the off axis magnetic field lines converging somewhat from magnet 21 to magnet 16. The enclosing vacuum chambet and other supporting equipment are not shown in this figure. Cesium vapor was introduced between cathode 40 and anode 42 at a flow rate of 20 milligrams per second. The are voltage was 42.5 volts and the corresponding current was amperes. An open circuit voltage of 21 volts was measured between shield 20 and anode 42 and the equivalent source resistance was determined to be a constant 8.6 ohms by varying load resistor 25 from 0 to infinity. When the current to magnet 21 was increased approximately 50% reducing the field convergence between magnet 21 and magnet 16 and expanding the plasma column in the vicinity of magnet 16, the open circuit voltage measured between shield 20 and anode 42 was reduced to 17.5 volts but the equivalent generator source impedance was reduced to 4.2 ohms and a greater amount of power could be extracted from shield 20. This illustrates the relation which exists between the size of the plasma column and the diameter of the collecting electrode. Similarly, increasing the cesium flow rate decreased the equivalent generator imp edance without markedly effecting the open circuit voltage. Thus, a cathode-shield voltage was obtained which was about of the applied cathode-anode voltage and substantial amounts of the cathode anode input power was dissipated through load resistor 25.

While the present invention has been particularly described in terms of specific embodiments thereof it will be understood that in view of the present disclosure numerous modifications thereof and deviations therefrom may now be readily devised by those skilled in the art without yet departing from the present teaching. Accordingly, the present invention is to be broadly construed and limited only by the spirit and scope of the claims now appended hereto.

What is claimed is:

1. In a plasma containment apparatus comprising a chamber, means to evacuate said chamber, magnetic means to form a longitudinally continuous magnetic field along a line within said chamber, at least one plasma arc generator disposed within said magnetic field on said line and substantially symmetrical thereabout, said generator comprising a central cathode electrode and an anode electrode encircling said cathode, said generator including at least one passage terminating between said cathode and anode, gas supply means to introduce a plasma forming gas through said passage, and power supply means to maintain an arc discharge between said anode and said cathode, a method of extracting electrical energy from the plasma column comprising:

intercepting with an electrode plasma at a potential higher than that of said anode and connecting an electrical load device between said electrode and said anode.

2. The method of claim 1 including intercepting said plasma with multiple electrodes each electrode being connected to an electrical load device.

3. Plasma containment apparatus comprising:

(a) a chamber;

(b) means to evacuate said chamber;

(c) magnetic means to form a longitudinally continuous magnetic field along a line within said chamber;

(d) at least one plasma arc generator disposed within said magnetic field on said line and substantially symmetrical thereabout, said generator comprising a central cathode electrode and an anode electrode encircling said cathode, said generator including at least one passage terminating between said cathode and anode;

(e) gas supply means to introduce a plasma forming gas through said passages;

(f) power supply means to maintain an arc discharge between said anode and said cathode;

(g) at least one annular electrode encircling the plasma column formed within said magnetic field and having an inner radius smaller than said plasma column; and,

(h) an electrical load device connected between said annular electrodes and said anode.

4. The apparatus of claim 3 in which more than one of said annular electrodes are employed.

5. Apparatus according to claim 3 wherein said magnetic means comprises an electrically insulated magnet coil enclosed within an electrically condutive shield, said shield being identical with said annular electrode.

6. Apparatus according to claim 5 wherein multiple shielded magnetic coils are employed.

7. Plasma containment apparatus comprising:

(a) a vacuum chamber;

(b) first vacuum pumping means to evacuate said vacuum chamber;

(0) magnetic means to form a longitudinally concontinuous magnetic field along a line within said chamber;

(d) at least one plasma arc generator disposed within said magnetic field on said line and substantially symmetrical thereabout, said generator comprising a central cathode electrode and an anode electrode encircling said cathode, said generator including at least one passage to introduce plasma forming gas through said passage, and power supply means to maintain an arc discharge between said anode and said cathode;

(e) annular flow restrictor means axially positioned on said line intermediate said first pumping means and said generator, said restrictor means being electrically conductive at least at the surface adjoining the inner radius thereof, said restrictor means being sealed to but electrically insulated from the walls of said vacuum chamber, said restrictor means having internal apertures smaller than the column formed by said plasma in said magnetic field;

(f) electrical load devices connected between said flow restrictor means and said anode; and,

(g) individual pumping means associated with each of said flow restrictors and positioned to evacuate volumes of said chamber adjacent the side of said restrictor in closest proximity to the nearest of said are generators.

References Cited UNITED STATES PATENTS 2,884,550 4/1959 Lalferty 324-33 X 3,104,345 9/1963 Wilcox et a1 315111 3,300,717 1/1967 Kemp et al. 324-72 DAVID X. SLINEY Primary Examiner 

1. IN A PLASMA CONTAINMENT APPARATUS COMPRISING A CHAMBER, MEANS TO EVACUATE SAID CHAMBER, MAGNETIC MEANS TO FORM A LONGITUDINALLY CONTINUOUS MAGNETIC FIELD ALONG A LINE WITHIN SAID CHAMBER, AT LEAST ONE PLASMA ARC GENERATOR DISPOSED WITHIN SAID MAGNETIC FIELD ON SAID LINE AND SUBSTANTIALLY SYMMETRICAL THEREABOUT, SAID GENERATOR COMPRISING A CENTRAL CATHODE ELECTRODE AND AN ANODE ELECTRODE ENCIRCLING SAID CATHODE, SAID GENERATOR INCLUDING AT LEAST ONE PASSAGE TERMINATING BETWEEN SAID CATHODE AND ANODE, GAS SUPPLY MEANS TO INTRODUCE A PLASMA FORMING GAS THROUGH SAID PASSAGE, AND POWER SUPPLY MEANS TO MAINTAIN AN ARC DISCHARGE BETWEEN SAID ANODE AND SAID CATHODE, A METHOD OF EXTRACTING ELECTRICAL ENERGY FROM THE PLASMA COLUMN COMPRISING: INTERCEPTING WITH AN ELECTRODE PLASMA AT A POTENTIAL HIGHER THAN THAT OF SAID ANODE AND CONNECTING AN ELECTRICAL LOAD DEVICE BETWEEN SAID ELECTRODE AND SAID ANODE. 